Scroll-type fluid displacement devices are well known. For example, U.S. Pat. No. 801,182 to Leon Creux, discloses a scroll device including two scroll members, each having a circular end plate and a spiroidal or involute scroll element. The scroll elements have identical, spiral geometry and are interfit with an angular and radial offset to create a plurality of line contacts between their spiral curved surfaces. Thus, the interfit scroll elements define and seal off at least one pair of fluid pockets. By orbiting one scroll element relative to the other, the line contacts are shifted along the spiral-curved surfaces, thereby changing the volume of the fluid pockets. This volume increases or decreases depending upon the direction of the scroll elements' relative orbital motion. Thus, the device may be used either to compress or expand fluids.
Known scroll-type fluid displacement devices, whether operating as expanders or compressors, can be used as vacuum pumps. However, both face a substantial potential for overheating.
Where an expander is used as a vacuum pump, ambient air will re-expand to the discharge pockets because the air pressure in the discharge pockets is much lower than the ambient air pressure. Re-expansion of ambient air in this fashion consumes energy and frequently causes overheating. A discharge valve can be employed to reduce re-expansion of the ambient air to some extent, but, it cannot eliminate re-expansion and such valves frequently malfunction.
When a compressor is used as a vacuum pump and the inlet air of the compressor is at atmospheric pressure during the start-up period, or due to leakage to ambient, the heat associated with the re-expansion and compression process is damaging to the compressor because there usually is no lubrication or internal cooling allowed. The re-expansion and compression heat causes excessive thermal growth of the scroll elements, resulting in galling between tips and bases of the scroll elements.
U.S. Pat. No. 3,994,636 discloses a tip seal mechanism for radial sealing between the compression pockets in a scroll-type fluid displacement device. In this device, as shown in the drawings as in FIG. 7, tip seals 101 and 201 are placed in spiral grooves 102 and 202 formed in the middle of the tips of a scroll vanes 103 and 203, respectively. These tip seals 101 and 201 run continuously along spiral grooves 102 and 202, from the central region to the periphery of the scroll members 103 and 203, respectively. The seals 101 and 201 are urged by either a mechanical device, such as elastic material, or by pneumatic force to contact the bases 204 and 104 of the other scroll member 203 and 103, respectively. This arrangement provides radial sealing. However, the width of the tip seal is smaller than the width of the scroll vane. There are tangential leakage passages A--A and B--B in scroll element 103, for example, at the both sides of the tip seal 101. These leakage passages lower the volumetric and energy efficiency of the scroll device.